In waste water treatment processes, relatively large, high capacity fluid pumps are employed to pump sewage, waste water, etc. through the treatment process. In general, the pumps are located in sumps or reservoirs and are selectively energized as a function of the fluid flow rate into the sump. In most instances, a level sensor monitors the liquid level in the sump. The level sensor forms part of a pump control system which energizes the requisite number of pumps needed in order to maintain a predetermined level in the sump.
Most pumps used in the above described environment, include detection devices or circuitry for deenergizing the pump drive motor in the event of a malfunction such as a pump overload. The two most prevalent forms of protection currently used in the industry are: (1) thermal switches located in the drive motor housing which open if the motor exceeds a predetermined temperature and (2) current monitoring sensors which interrupt power to the drive motor should the drive motor current levels exceed a predetermined threshold.
In the first protection method, thermal responsive contacts respond quickly to an overload condition but because they are located in the pump motor housing which is often submerged in the sump, they cannot be manually reset and therefore in most instances, the contacts reclose after the pump motor has cooled below the overload temperature and the pump motor is automatically reenergized.
With the second protection method, the current sensor can be made part of a manual reset circuit to prevent restarting the pump motor until the circuit has been manually reset by the operator, thereby apprising the operator of the malfunction and allowing him a chance to investigate. However, the circuit has the inherent disadvantage of only "indirectly" sensing a motor overload. The current sensor is typically mounted at a remote location, the ambient temperature of which can differ substantially from the temperature of the pump environment. If the ambient temperature in the vicinity of the current sensor is substantially higher than the pump environment, "false trips" can occur. Conversely, if the ambient temperature is substantially lower than the pump environment, the current sensor may not open immediately, resulting in possible drive motor damage due to the delayed shutdown.
From the above discussion it should be apparent that a manual reset protection device that includes sensors located in or near the pump motor assembly which directly monitor pump temperature or other conditions, is desirable. The prior proposed devices for providing these desired features have either been ineffective or too costly.
Moisture sensors for monitoring pump seal integrity that are operative to deenergize the pump motor should a seal failure be imminent are also desirable. Like the thermal overload protection, it is desirable that once the pump motor is deenergized by the seal sensor, that it be reenergized only after the circuit has been manually reset, so that the operator is apprised of the malfunction and given an opportunity to take corrective action if warranted to prevent further damage to the pump motor assembly.